In the past, supports for helically wound fusible elements have been made either of ceramic like non gassing materials or materials which decompose, evolving gases when subjected to heat.
A major disadvantage to supports of ceramic like material is that they occupy a relatively large volume within the fuse casing, which would otherwise be occupied by arc quenching filler material, without themselves contributing to the arc quenching process. On the other hand, supports made solely of gas evolving material suffer the disadvantage of producing gas both in excess of that required for arc quenching and also in excess of that which the fuse casing is able to withstand. Consequently, the high pressure associated with the gas ruptures the casing.
Currently, supports are made which are essentially a compromise of the two above mentioned approaches. That being a support of ceramic or other non gas evolving material being either partially constructed of gas evolving material or having inserts or clips of gas evolving material affixed thereto. These supports are constructed so that the fusible element contacts the gas evolving portions only in localized and predetermined regions, to thereby limit the volume of gas produced to a tolerable level. Examples of this type of support are described in U.S. Pat. Nos. 3,925,745 issued to Cameron on Dec. 9, 1975 and 4,099,153 issued to Cameron on July 4, 1978. In the above mentioned patents, the apparatus described therein depends on physical contact between the fusible element and the gas evolving material. Such an arrangement has been found to have one serious disadvantage, being that the gas evolving material is constantly subjected to heat from the fusible elements during normal, high-normal and overload operating conditions. The result being a gradual decomposition of the gas evolving material at temperatures well below the fusing temperature of the element. In such cases, the gas evolving material may become seriously decomposed or in fact, may be totally decomposed before such time as the element actually fuses and an arc is struck. Other variations of supports are disclosed in copending U.S. applications Ser. No. 883,810 filed Mar. 16, 1978, now U.S. Pat. No. 4,183,004 and Ser. No. 895,536 filed Apr. 12, 1978, now U.S. Pat. No. 4,183,004 both by Kozacka and assigned to the same assignee as the assignee of the present invention.
It would be advantageous to have a fuse in which there was a support which satisfied the requirements structurally and insulatively and which carried gas evolving means which remained essentially unaffected by fuse operating temperatures below the actual fusing temperature.